Chemical Fixation of Carbon Dioxide has attracted much attention in view of environmental, legal, and social issues in the past few decades due to increasing levels of carbon dioxide. Carbon dioxide is an attractive C1 building block in organic synthesis because it is an abundant, renewable carbon source and an environmentally friendly chemical reagent. The utilization, as opposed to the storage of CO2 is indeed more attractive especially if the transformation of carbon dioxide to useful bulk products is an economical one. Much research has been devoted toward exploring technologies for CO2 transformation, whereby harsh and severe reaction conditions are one of the major limitations for their practical applications. Therefore, the development of efficient catalyst systems for CO2 utilization under mild conditions is highly desired, especially for real world applications.
Carboxylic acids are one of the most important types of compounds in medicinal chemistry and also in fine-chemicals synthesis. Although there are many well-established protocols for the preparation of carboxylic acids, the direct carboxylation of carbon nucleophile using CO2 as the electrophile is the most attractive and straightforward method. The formation of a stable C—C bond is desired for CO2 fixation and remains the most challenging aspect thus far. Typically, this type of reaction is facilitated by the insertion of CO2 into a metal-carbon bond in presence of organometallic reagents. Widespread use of these processes is limited by the synthesis of related organometallic reagents as precursors and the restricted substrate scope.
In the past decades, several interesting systems have been reported for metal-mediated reductive carboxylation of alkenes, alkynes to form carboxylic acids or esters. There are few methods available in the literature for direct carboxylation of terminal alkynes.
Carboxylation of terminal alkynes by using silver and copper catalyst under homogeneous conditions is known in the art.
WO2011075087 titled “Carboxylation of Terminal Alkynes” describes a process for converting a terminal alkyne into an alkynoic acid. In the process, alkyne is exposed to carbon dioxide in the presence of a copper (I) species, a base and a complexing agent such as diaminoalkene, N-heterocyclic carbene capable of complexing copper (I).
An article titled “The direct carboxylation of terminal alkynes with carbon dioxide” by Yu Dingyi; Zhang Yugen in Green Chemistry (May 2011), 13 (5), pg. 1275-1279 discloses direct carboxylation of terminal alkynes using CO2 as the C1 carbon feedstock. The direct C—H bond functionalization is achieved with Cs2CO3 as the base and in the absence of transition metal catalyst. However, the process employs high pressure and temperature.
The processes in the art suffer from certain drawbacks like use of expensive or complex ligands to guarantee selectivity and catalytic efficiency. Further, the processes need either a stoichiometric amount of transition metals as reactants or an excess amount of organometallic reagents for trans metallation.
Further, as environmental regulations and safety concerns are the burgeoning issues faced by the industrial society today, development of environmentally benign methodologies remains the key issue. Among the viable alternatives available for green synthetic methods, clays and clay-based catalysts in particular have attracted significant attention due to their extremely versatile properties. Due to their structural features, they can easily be modified with different metal cations, or organic/organometallic compounds resulting in new catalysts of potential importance. Both the modified and natural clays can be applied to catalyze a broad variety of chemical transformations, thus providing exceptional importance for these materials in the development of new synthetic processes.